This invention is generally in the area of the Fischer-Tropsch synthesis.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed which can produce fuel compositions from methane. Most of these process involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes a broad spectrum of products, ranging from methane to wax, which includes a significant amount of hydrocarbons in the distillate fuel range (C5-20).
Methane tends to be produced when chain growth probabilities are low. The methane can be recirculated through the syngas generator, but minimizing methane formation is generally preferred. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. The wax can be processed to form lower molecular weight products.
The hydrocarbons in the distillate fuel range are mostly linear, and tend to have relatively low octane values, relatively high pour points and relatively low sulfur contents. They are often isomerized to provide products with desired octane and pour point values.
Many isomerization catalysts require low levels of sulfur and nitrogen impurities, and feedstreams for these catalysts are often hydrotreated to remove the sulfur and nitrogen compounds. Feeds to be isomerized are often contacted with a sulfur-tolerant catalyst in the presence of hydrogen to minimize the amount of sulfur in the feed.
When isomerization processes are carried out with un-sulfided catalysts, various side reactions, such as hydrogenolysis (hydrocracking), can occur, producing undesired C1-4 hydrocarbons. Such hydrogenolysis can be supressed by incorporating a small amount of sulfur-containing compounds into the feed, or by using other hydrocracking suppressants.
It would be advantageous to provide an efficient process for isomerizing the hydrocarbons in the distillate fuel range from Fischer-Tropsch syntheses that minimizes the amount of hydrogenolysis. The present invention provides such a process.
An integrated process for producing a hydrocarbon stream including C5-20 normal and iso-paraffins is disclosed. The process involves isolating a methane stream from a natural gas source, wherein the methane stream is treated to remove sulfur-containing impurities. A C5+ stream is also isolated from the natural gas source, wherein the C5+ stream includes sulfur-containing impurities. At least a portion of the methane stream into converted into syngas, and the syngas is subjected to a hydrocarbon synthesis process, for example, Fischer-Tropsch synthesis, to produce a product stream including C5-20 hydrocarbons, among other products. The C5-20 stream is then isolated, for example, by fractional distillation or solvent extraction.
At least a portion of the C5-20 stream from the syngas reaction is combined with at least a portion of the C5+ stream from the natural gas source. The combined streams are subjected to hydroprocessing conditions which involve hydrotreating and hydroisomerizing the hydrocarbons over an acidic catalyst. At least one of the catalyst components is a pre-sulfided catalyst, for example, a pre-sulfided Group VIII non-noble metal or tungsten catalyst.
The sulfur compounds present in the C5+ stream act as a hydrocracking suppressant, and minimize the amount of hydrocracking (hydrogenolysis) which would otherwise occur during the hydroprocessing reaction and form undesired C4-products. After the hydroprocessing step, any remaining sulfur compounds can be removed, for example, using adsorption, extractive Merox or other means well known to those of skill in the art.
The hydroprocessing catalysts can include cobalt and/or molybdenum in catalytically effective amounts. The acidic component can be a silica-alumina support, where the silica/alumina ratio (SAR) is less than 1 (wt./wt.). For pre-sulfided catalysts, the amount of sulfur is typically between about 0.1 and 10 wt %.
In one embodiment, Fischer-Tropsch wax products are also isolated, and are treated to provide a C5-20 product stream. This stream can also be hydroprocessed in combination with at least a portion of the C5+ stream from the natural gas and, optionally, in combination with at least a portion of the C5-20 product stream from the Fischer-Tropsch synthesis.
In another embodiment, at least a portion of the C2-4 products from the Fischer-Tropsch reaction are subjected to further processing steps, for example olefin oligomerization, to provide an additional C5-20 product stream. This product stream can also be hydroprocessed in combination with at least a portion of the C5+ stream from the natural gas, and, optionally, in combination with at least a portion of the C5-20 product stream from the Fischer-Tropsch synthesis and/or the product stream resulting from the processing of the Fischer-Tropsch wax.
The processes described herein significantly reduce hydrogenolysis, resulting in a significant increase in the overall yield of C5-20 hydrocarbons.